Global Sea Level Reconstruction for 1900-2015 Reveals Regional Variability In Ocean Dynamics and an Unprecedented Long Weakening in the Gulf Stream Flow Since the 1990s

A new monthly global sea level reconstruction for 1900-2015 was analyzed and compared with various observations to examine regional variability and trends in the ocean dynamics of the western North Atlantic Ocean and the U.S. East Coast. A proxy of the Gulf Stream (GS) strength in the Mid-Atlantic Bight (GS-MAB) and in the South Atlantic Bight (GS-SAB) were derived from sea level differences across the GS in the two regions. While decadal oscillations dominate the 116-year record, the analysis showed an unprecedented long period of weakening in the GS flow since the late 1990s. The only other period of long weakening in the record was during the 1960s-1970s. Ensemble Empirical Mode Decomposition (EEMD) was used to separate oscillations at different time scales, showing that the low-frequency variability of the GS is connected to the Atlantic Multidecadal Oscillations (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The recent weakening of the reconstructed GS-MAB was mostly influenced by weakening of the upper mid-ocean transport component of AMOC as observed by the RAPID measurements for 2005-2015. Comparison between the reconstructed sea level near the coast and tide gauge data for 1927-2015 showed that the reconstruction underestimated observed coastal sea level variability for time scales less than ~ 5 years, but lower frequency variability of coastal sea level was captured very well in both amplitude and phase by the reconstruction. Comparison between the GS-SAB proxy and the observed Florida Current transport for 1982-2015 also showed significant correlations for oscillations with periods longer than ~ 5 years. The study demonstrated that despite the coarse horizontal resolution of the global reconstruction (1°x1°), long-term variations in regional dynamics can be captured quite well, thus making the data useful for studies of long-term variability in other regions as well

Effects of Seasonal MSL Variability on Extreme Sea Levels in the German Bight

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Long-term variations in global sea level extremes

Decadal to multidecadal variations in sea level extremes unrelated to mean sea level changes have been investigated using long tide gauge records distributed worldwide. A state space approach has been applied that provides robust solutions and uncertainties of the time evolving characteristics of extremes, allowing for data gaps and uneven sampling, both common features of historical sea level time series. Two different models have been formulated for the intensity and for the occurrence of extreme sea level events and have been applied independently to each tide gauge record. Our results reveal two key findings: first, the intensity and the frequency of occurrence of extreme sea levels unrelated to mean sea level vary coherently on decadal scales in most of the sites examined (63 out of 77) and, second, extreme sea level changes are regionally consistent, thus pointing toward a common large-scale forcing. This variability of extremes associated with climate drivers should be considered in the framework of climate change studies

Long-term variations in global sea level extremes

Decadal to multidecadal variations in sea level extremes unrelated to mean sea level changes have been investigated using long tide gauge records distributed worldwide. A state space approach has been applied that provides robust solutions and uncertainties of the time evolving characteristics of extremes, allowing for data gaps and uneven sampling, both common features of historical sea level time series. Two different models have been formulated for the intensity and for the occurrence of extreme sea level events and have been applied independently to each tide gauge record. Our results reveal two key findings: first, the intensity and the frequency of occurrence of extreme sea levels unrelated to mean sea level vary coherently on decadal scales in most of the sites examined (63 out of 77) and, second, extreme sea level changes are regionally consistent, thus pointing toward a common large-scale forcing. This variability of extremes associated with climate drivers should be considered in the framework of climate change studies.M. Marcos acknowledges a “Ramon y Cajal” contract funded by the Spanish Ministry of Economy. This work was supported by the research project CLIMPACT (CGL2014-54246-C2-1-R) funded by the Spanish Ministry of EconomyPeer Reviewe

Editorial: Sea level variability and change

Peer reviewedPeer Reviewe

Effects of the Temporal Variability of Storm Surges on Coastal Flooding

Assessments of flood exposure and risk are usually conducted for individual events with a specific peak water level and hydrograph, without considering variations in the temporal evolution (duration and intensity) of storm surges. Here we investigate the influence of temporal variability of storm surge events on flood characteristics in coastal zones, namely flood extent and inundation depth, and assess the associated flood exposure in terms of affected properties for the case of the municipality of Eckernförde, Germany. We use a nested hydrodynamic model to simulate five physically plausible, stochastically simulated storm surge events, with peak water levels corresponding to a univariate return period of 200 years and varying intensities. In a second step, the events are also combined with high-end sea-level rise projections corresponding to the RCP 8.5 scenario to analyze if the influence of temporal variability changes with rising sea-levels. Results show differences exceeding 5% in flood extent when comparing storm surges with the highest and lowest intensities. The number of properties exposed differs by approximately 20%. Differences in mean and maximum inundation depths are approximately 5%, both with and without sea-level rise. However, deviations in flood extent increase by more than 20%, depending on the sea-level rise projection, whereas differences in the number of exposed properties decrease. Our findings indicate that the temporal variability of storm surges can have considerable influence on flood extent and exposure in the study area. Taking into account that flood extent increases with rising sea-levels, we recommend that uncertainty related to the temporal variability of storm surges is represented in future flood risk assessments to ensure efficient planning and to provide a more comprehensive assessment of exposed infrastructure and assets

Circulation,Volume 26, No.1

Fall 2020 issue of CCPO Circulation featuring the article, Understanding the Causes and Impact of Sea Level Rise, by Sönke Dangendorf.https://digitalcommons.odu.edu/ccpo_circulation/1064/thumbnail.jp

Mean Sea Level Changes in the Southwestern Baltic Sea Over the Last 190 Years

(First paragraph) Over the 20th century a global mean sea level (GMSL) rise of about 1.3 to 2 mm/yr could be observed and it is projected to further accelerate throughout the 21st century (Church and White 2006, Hay et al., 2015; Dangendorf et al., 2017). However, GMSL rise is neither temporally nor spatially uniform. Because of a number of different factors (e.g. mass changes and gravitational effects due to melting ice sheets/glaciers, expanding/contracting volume due to temperature and salinity fluctuations, ocean circulation changes, atmospheric forcing), regional mean sea level (MSL) trends can vary significantly from the global average. In order to develop sustainable coastal protection strategies, local/regional sea level studies are necessary

Mean sea level variability and influence of the North Atlantic oscillation on long-term trends in the German Bight

Changes in the seasonal cycle of mean sea level (MSL) may affect the heights of storm surges and thereby flood risk in coastal areas. This study investigates the intra- and inter-annual variability of monthly MSL and its link to the North Atlantic Oscillation using records from 13 tide gauges located in the German Bight. The amplitudes of the seasonal MSL cycle are not regionally uniform and vary between 20 and 29 cm. Generally, the amplitudes are smaller at the southwestern stations, increasing as one travels to the northeastern part. The amplitudes, as well as the phase of the seasonal cycle, are characterized by a large inter-annual and inter-decadal variability, but no long-term trend could be detected. Nevertheless, in the last two decades annual maximum peaks more frequently occurred in January and February, whereas beforehand an accumulation was detected for the November and December period. These changes in phase in the various sea level time series are consistent with a shift in the annual cycle, which is, however, not significant. The changes are associated with strongly increasing trends in monthly MSL of the winter season (J–M), which are considerably higher compared to the remaining seasons. For the same season, the MSL and North Atlantic Oscillation (NAO) indices show strong similarities, resulting in statistically significant correlations (r ~ 0.7). Hence, these changes are linked with changing pressure conditions over the North Atlantic, which lead to a strong phase of positive values in the NAO index between the 1960’s and 1990’s